The present invention relates to a piezoelectric filter having an improved group delay time characteristic (hereinafter referred to as the G.D.T. characteristic).
There has heretofore been known a ladder type filter constituted by a plurality of piezoelectric resonators each utilizing a face vibration mode or a radial vibration mode, an example of which is shown in FIG. 1 of the accompanying drawings in side elevational view. Referring to FIG. 1, the prior art ladder type filter comprises a plurality of, for example, five, piezoelectric resonators 1a, 1b, 1c, 1d and 1e. Of these piezoelectric resonators, the piezoelectric resonators 1a, 1c and 1e are series resonance elements while the piezoelectric resonators 1b and 1d are parallel resonance elements, these piezoelectric resonators being so arranged that a parallel resonance element is positioned between each two adjacent series resonance elements. These piezoelectric resonators 1a to 1e are of identical construction and each piezoelectric resonator 1a, 1b, 1c, 1d and 1e includes a piezoelectric substrate of a plate-like configuration of any desired shape, for example, a square shape, having its opposing major surfaces coated or deposited with electrode layers of known fabrication, and first and second electroconductive terminal plates 2a and 3a, 2b and 3b, 2c and 3c, 2d and 3d, or 2e and 3e. Each of these first and second terminal plates contacts and is, therefore, electrically connected to a corresponding face of the piezoelectric substrate through an electroconductive protuberance 4a rigidly secured to or integrally formed with the terminal plates, the point of contact of the protuberance 4a with the corresponding face of the piezoelectric substrate of the associated resonator corresponding to a node of vibration of that piezoelectric substrate. The protuberances 4a are kept at a certain position and are urged against the corresponding face of the piezoelectric substrate of the associated resonator by application of a small pressure, such as 250.about.300 g/cm.sup.2 by means of a spring having a known construction. While the terminal plates 2a and 3e serve as input and output terminal elements, respectively, the terminal plates 3a and 2c are electrically connected to each other by means of a bridge element 5 and the terminal plates 3c and 2e are electrically connected to each other by means of a bridge element 6. Electrically insulating sheets 7 and 8 are positioned respectively between the terminal plates 3b and 2c and between the terminal plates 3d and 2e. In use, the terminal plates 3b and 3d are electrically grounded.
The selective filtering characteristic and the G.D.T. characteristic of the prior art ladder type filter are shown in a graph of FIG. 2. An improvement of such a ladder type filter as described above has long been desired to improve the G.D.T. characteristic in order to suppress the phase distortion. One conventional method for improving the G.D.T. characteristic is to decrease the mechanical quality factor Q to a value of from several tens to hundreds. However, it has been found that the conventional method for improving the G.D.T. characteristic entails various difficulties. In particular, depending upon the particular value of the mechanical quality factor Q chosen, the ratio of materials to be mixed together has to be adjusted and, consequently, the filter having a desired temperature characteristic TC, electromechanical coupling coefficient K and dielectric constant can no longer be obtained without difficulty.
Moreover, as the mechanical quality factor Q is lowered, the frequency constant (Fol) is correspondingly reduced. By way of example, when a filter utilizing resonators having a relatively low mechanical quality factor Q is incorporated in a casing in which resonators having a mechanical quality factor Q of about 1,000 are incorporated, the resistance to impact tends to be intolerably low, and the attentuation level varies considerably, because the resonators employed are small in size.